1. Field of the Invention
The present invention relates generally to processes for semiconductor manufacturing and, more particularly, to optical lithography techniques for the determination of focal plane deviation (FPD) associated with photolithographic projection systems.
2. Description of the Related Art
Semiconductor manufacturers and lithography tool vendors have been forced to produce higher numerical aperture (NA) lithography systems (steppers or scanners) using smaller wavelengths (for example, 193 nm DUV lithography) in response to the semiconductor industry's requirement to produce ever-smaller critical features. See, for example, the statement of the well-known “Moore's Law” at “Cramming more components onto integrated circuits”, G. Moore, Electronics, Vol. 38, No. 8, 1965. The ability to produce (manufacture) sub-wavelength features can often be determined by considering the rather simple (3-beam) Rayleigh scaling Resolution (R) and (Reference A) Depth-of-Focus (DoF) equations; ˜λ/2NA and ˜λ/2NA2 respectively. These coupled equations stress the inverse relationship between resolution and DoF based on the exposure wavelength (λ and numerical aperture (NA) for features printed near the limit of the optical system. High NA lithography has led to improved resolution and a reduction in the overall focus budget, making lithography processes difficult to control. See “Distinguishing dose from defocus for in-line lithography control”, C. Ausschnitt, SPIE, Vol. 3677, pp. 140–147, 1999; and “Twin Scan 100 Product Literature”, ASML). Poor lithographic process control (focus and exposure) leads to smaller product yields, increased manufacturing costs, and poor time to market. While semiconductor lithographers have discovered creative reticle enhancement techniques (RETs) and other optical techniques (PSM) to increase the useable DoF—the problem remains. See, for example, “The Attenuated Phase-Shifting Mask”, B. Lin, and “Method and Apparatus for Enhancing the Focus Latitude in Lithography”, Pei-Yang Yan, U.S. Pat. No. 5,303,002, Apr. 12, 1994. Therefore, it is crucial to monitor focus during photolithographic processing and develop new methods for focus control. Typically focus error across a scanner field can be attributed to the following three factors: 1) wafer and reticle non-flatness, 2) dynamic wafer/reticle stage error, and 3) static and/or dynamic lens field curvature. For a photolithographic scanner, dynamic field curvature varies in the cross scan direction (x) in rather complex ways.
The ability to precisely control the photolithographic scanner tool depends on the ability to determine the magnitude and direction of the individual focusing error components (items 1–3 above) and to account for repeatable and non-repeatable portions of those errors. While focusing error causes reduction in image fidelity, the coupling of focus error and other lens aberrations (distortions) degrades overlay or positional alignment as well. See, for example, “Impact of Lens Aberrations on Optical Lithography”, T. Brunner.
Over the past 30 years the semiconductor industry has continued to produce faster (via smaller critical features) and more complex (greater functionality, dense patterning) circuits, year after year. See, for example, “Optical Lithography—Thirty years and three orders of magnitude”, J. Bruning, SPIE, Vol. 3051, pp. 14–27, 1997; and “Cramming more components onto integrated circuits”, supra. The push to smaller feature sizes is gated by many physical limitations. See “Introduction to Microlithography”, L. Thompson et al., ACS 2nd Edition, p. 69, 1994. As the critical dimensions of semiconductor devices approach 50 nm, the usable DoF will approach 100 nm. See “2001 ITRS Roadmap”, SEMATECH, pp. 1–21). Continued advances in lithography equipment (higher NA systems, smaller wavelength exposure sources), RET's, resist processing, and automated process (focus and exposure) control techniques will get more difficult and remain critical. See, for example, “2001 ITRS Roadmap”, supra; and “The Waferstepper Challenge: Innovation and Reliability despite Complexity”, Gerrit Muller, Embedded Systems Institute Netherlands, pp. 1–11, 2003. Finally, while FPD deviation can be determined using a variety of methods, none of these methods have the ability to divide the focal error into correctable (possibly systematic) and non-correctable (possibly random) portions—especially for scanners and to further decouple the effects of wafer flatness. The ability to decouple focus error leads directly to improved dynamic scanning behavior using a variety of advanced process control techniques. See “Predictive process control for sub-0.2 um lithography”, T. Zavecz, SPIE ML, Vol. 3998–48, pp. 1–12, 2000; “TWINSCAN 1100 Product Literature”, supra; and “Advanced statistical process control: Controlling sub-0.18 μm Lithography and other processes”, A. Zeidler et al., SPIE, Vol. 4344, pp. 312–322, 2001.
It should be noted that, even if a perfect lens with no dynamic lens field curvature (ZL=0) could be obtained, the lens could still be associated with FPD due to scanner dynamic focal plane deviation (SFPD), which is the scanner field curvature error associated with stage synchronization error in the Z direction. Thus, in view of the industry trends described above, more precise techniques for determining FPD and SFPD are continuously desired.
FPD: There are a number of methods that with greater or lesser accuracy measure defocus or focal plane deviation (FPD) over an exposure field. In general terms, each of these techniques estimate the focal error across the field using a variety of special reticle patterns (focusing fiducials, FF), interferometric devices, mirrors, sensors, and statistical models. In addition, each of these methods utilizes the stepper or scanner wafer stage leveling and positioning system and/or optical alignment system to aid in the determination of FPD. See, for example, “TWINSCAN 1100 Product Literature”, supra. The term “FPD” is a rather general term describing the complete focus error associated with the photolithographic stepper or scanner, deviations from the focal plane in reference to the wafer surface. Among other things, FPD can be caused by lens tilt, stage/reticle tilt, reticle bow, lens field curvature, and stage synchronization error. FIG. 7 shows a generic photolithographic leveling system. FIG. 8 illustrates some common reticle patterns (e.g. IBM's Phase Shift Focus Monitor (PSFM), and ASML's FOCAL alignment mark) that are used to determine FPD for both steppers and scanners. Typically, FPD calibration/monitoring is performed daily or at least weekly to ensure that the stepper or scanner is operating within design limits (verifying the focus system works, the stage is level, etc.). While both techniques are widely accepted both techniques require complex calibrations to be performed at each field point. See “Detailed Study of a Phase-Shift Focus Monitor”, G. Pugh et al., SPIE Vol. 2440, pp. 690–700, 1995; and “FOCAL: Latent Image Metrology for Production Wafer Steppers”, P. Dirksen et al., SPIE, Vol. 2440, p. 701, 1995).
Table 1 below lists some FPD prior art methods:
TABLE 1MeasurementMethodTypeCommentISI (See “Apparatus,AbsoluteExtremelyMethod of Measurement andaccurate.Method of Data Analysis forCorrection of OpticalSystem”, A. Smith et al.,U.S. Pat. No. 5,828,455 issuedOct. 27, 1998 and “Apparatus,Method of Measurement andMethod of Data Analysis forCorrection of Optical System”,A. Smith et al., U.S. Pat. No.5,978,085 issued Nov. 2, 1999)FOCAL (See “FOCAL: LatentRelativePublishedImage Metrology for Productionversion claimsWafer Steppers”, supra)high absoluteaccuracy,resolutionaveraging inpractice.IBM focus monitor (SeeAbsoluteRequires“Optical Focus Phase Shift‘calibration’.Test Pattern, Monitoring SystemIt is veryand Process”, T. Brunner etprocessal., U.S. Pat. No.independent.5,300,786 issued Apr. 5, 1994)Schnitzl (See “DistinguishingRelativeComplexDose from Defocus for In-Linewith onecalibration,Lithography Control”, supra)exposurevarying targetsensitivity.TIS (See “193 Step and ScanRelativeRelies on waferLithography”, G. Davies etZ-stage,al., Semi Tech Symposium, Japan,accuracy/repeat.1998; and “Twin Scan 1100Product Literature”, supra)
ISI (Litel): A method for determining the aberrations of an optical system is described in U.S. Pat. No. 5,828,455, supra, and U.S. Pat. No. 5,978,085, supra. Where a special reticle is used to determine the Zernike coefficients for photolithographic steppers and scanners. Knowing the wavefront aberration (Zernike coefficients and the associated polynomial) associated with the exit pupil of the projection system includes information about the lens field curvature or focus (Zernike coefficient a4, for example). Smith uses a special reticle and a self-referencing technique to rapidly identify FPD to a high degree of accuracy, determines focusing errors to ˜5 nm, in the presence of scanner noise. This method automatically determines lens field curvature information for both static and dynamic exposure tools (steppers and scanners).
PSFM: A method (Phase Shift Focus Monitor) described in U.S. Pat. No. 5,300,786, supra, can be used to determine and monitor the focal plane deviation (FPD) associated with the lithographic process. More information can be found in “Detailed Study of a Phase-Shift Focus Monitor”, supra. In general, an alternating PSM with phase close to 90° possesses unusual optical properties that can be exploited to measure focus errors. See, for example, “Quantitative Stepper Metrology Using the Focus Monitor Test Mask”, T. Brunner et al., SPIE, Vol. 2197, pp. 541–549; and “Using the Focus Monitor Test Mask to Characterize Lithographic Performance”, R. Mih et al., SPIE, Vol. 2440, pp. 657–666, 1995. It is possible to design a “box-in-box” overlay target using a phase shift mask pattern (referred to here as a focusing fiducial; see FIGS. 8–9), in which the measured overlay error is proportional to the focus error (see FIG. 10). Focal plane non-flatness is then determined by measuring the focusing fiducials across the lens field. Astigmatism information appears as differences between the delta-X overlay error and the delta-Y overlay error measurement. This technology has also been used for assessing variations in focus across the wafer due to lens heating, misfocusing near the edge of the wafer, and chuck/stage non-flatness. One major drawback with the PSFM method is that a fairly elaborate calibration procedure (focus offset vs. overlay shift for each field point) is required before it can be used, the PSFM technique is rather sensitive to the source-sigma (Na-source/Na-objective) that varies from process to process. Additional PSM techniques, such as those found in “Monitor for Alternating Phase Shift Masks”, L. Liebmann et al., U.S. Pat. No. 5,936,738 issued Aug. 10, 1999, are used in a similar way. While the PSFM method provides an FPD map across a scanner or stepper field it does not provide a method for determining the dynamic lens field curvature independent of wafer height variation in the presence of stage synchronization error. See, for example, “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, Dusa et al., SPIE, Vol. 2726–29, 1996.
FOCAL: A method (FOCAL—Focus determination using stepper alignment system) described by P. Dirksen, et. al., SPIE, Vol. 2440, 1995, p. 701, specifies a focusing fiducial that can be used to find FPD and astigmatism across the exposure field (lens). FOCAL alignment marks (focusing fiducials) consist of modified wafer alignment marks that are measured using the stepper wafer alignment subsystem. See, for example, FIG. 1 of “FOCAL: Latent Image Metrology for Production Wafer Steppers”, P. Dirksen et al., SPIE, Vol. 2440, p. 701, 1995. Defocus of the tool results in an apparent shift of the center of the alignment mark relative to that of the ‘best focus’ position. The FOCAL technique makes use of the exposure tool's alignment mechanism and therefore requires that the stepper or scanner be off-line for the length of the measurement sequence. FOCAL marks are sensitive to exposure and sigma like the PSFM method; however, since fiducial response is a function of pitch, the target features are less dependent upon reticle error. Furthermore, the FOCAL data (focus vs. overlay error) must be calibrated for every point in the exposure field similar to phase-shift monitors (typically at 121 points across an exposure field, see FIG. 10). Now, it is possible to use FOCAL to separate out lens tilt and astigmatism from dynamic FPD maps and provide a dynamic focal plane map, but wafer height variation and stage synchronization errors would still be included in the result. See, for example, “193 Step and Scan Lithography”, supra; and “Comprehensive Focus-Overlay-CD Correlation to Identify Photolithographic Performance”, supra.
Schnitzl Targets: A method described by Ausschnitt in “Distinguishing Dose from Defocus for In-Line Lithography Control”, C. Ausschnitt, SPIE, Vol. 3677, pp. 140–147, 1999, makes use of line-end shortening effects to decouple focus drift from exposure drift on semiconductor product wafers. FIG. 9 shows a typical pair of Schnitzl targets (focusing fiducials). It is widely known that resist line-ends (FIG. 9) are very sensitive (exhibit greater line-end shortening) to both focus and exposure drifts; the effect is further enhanced as the lithographic process is pushed near performance limit of the scanner tool (˜λ/2NA). Using the Schnitzl targets and a fairly elaborate method of calibration (CD-SEM measurements and a coupled system of equations) Ausschnitt offers a method that can determine the magnitude of focus drift on product wafers using one or more exposures in the presence of exposure drift (see FIG. 10 for example results). Since changes in focus and exposure can produce similar changes in the critical dimension (CD) the Schnitzl method is useful for day-to-day process monitoring because it eliminates the need to constantly perform focus and exposure experiments (FEM—a Focus Exposure Matrix) in-between production runs. In addition, the method uses fast and accurate optical overlay tools to measure the Schnitzl patterns (in several forms, CD targets or Overlay targets, FIGS. 8–9) after wafer processing, this saves monitoring costs because optical overlay tools are less expensive to operate as compared with a CD-SEM. While decoupling focus drift from exposure drift is useful for process monitoring, the method in its present form requires two exposures at different focus settings to determine the absolute focal drift (direction). Performing extra exposures during production runs is very costly. In addition, since the initial Schnitzl target calibration procedure depends on a number of lithographic tool settings (line size, pitch, sigma, NA) re-calibration is required for each lithographic process change—including changes in metrology tools. The Schnitzl focusing fiducials are often used to map out FPD across a stepper or scanner field, but methods similar to those described in “Comprehensive focus-overlay-CD correction to identify photolithographic performance”, Dusa, et al., SPIE Vol. 2726–29, 1996, would need to be implemented to obtain a dynamic focus map—but again, wafer height variation and scanning dynamics are not considered.
Summarizing:
Several methods for determining FPD have been described. Common to all of these methods is that a feature (focusing fiducial or FF) is printed on a wafer and the focusing fiducial is subsequently measured. The data from the focusing fiducial is processed and an FPD value, δZ, is determined. Further, and common to all these methods, the contributions of wafer height, lens aberrations (in the form of lens field curvature), and stage synchronization are not resolved into their distinct components.